1. Field of the Invention
The present invention relates to a coating-type magnetic recording medium in which .gamma.--Fe.sub.2 O.sub.3 is used as a magnetic material, and more particularly to a micro-cassette tape for used in telephone-answering machine, tape recorders for conference system, or the like.
2. Prior Art
Conventionally, there have been extensively utilized coating-type magnetic recording media which are produced by applying a magnetic coating material onto a non-magnetic substrate and then drying the magnetic coating material. The magnetic coating material is prepared by dispersing a magnetic powder material such as an oxide-based magnetic powder or an alloy-based magnetic powder in an organic binder such as vinyl chloride-vinyl acetate-based copolymer, a polyester resin, an urethane resin, a polyurethane resin or the like.
Incidentally, as magnetic materials for micro-cassette tapes capable of long-term recording, such as 90 minute-recording micro-cassette tapes, the alloy-based magnetic powder have been generally used. The alloy-based magnetic powder has an extremely large coercive force and saturation magnetization as compared to the oxide-based magnetic powder. For this reason, the magnetic tape in which such an alloy-based magnetic powder is used as a magnetic material can exhibit a high coercive force and a high residual magnetic flux density.
However, since the alloy-based magnetic powder is a ferromagnetic metal alloy (Fe, Co, Ni) composed mainly of Fe, the magnetic powder exhibits a low stability against oxidation thereof, so that there arises a risk that the magnetic powder is burned or ignited upon handling. In addition, the alloy-based magnetic powder is apt to be coagulated during the production of the magnetic coating material. For this reason, when the alloy-based magnetic powder is used, the magnetic coating material suffers from the increase in viscosity thereof, thereby limiting the conditions for the production of magnetic tapes. Further, when the resultant magnetic tape is loaded in a tape recorder or the like and traveled therein, there also occurs a risk that a traveling reliability and durability of the magnetic tape are deteriorated due to the increase in frictional coefficient thereof.
In view of the afore-mentioned problems encountered in the prior art, the present inventors have paid their attention to .gamma.--Fe.sub.2 O.sub.3 because the material is excellent in chemical stability and magnetic stability and can produced with a low cost. However, .gamma.--Fe.sub.2 O.sub.3 is deteriorated in electromagnetic conversion characteristic as compared to the alloy-based magnetic powder. Further, in the case of the micro-cassette tape serving for a long-term recording, such as a 90 minute-recording micro-cassette tape, it is required that the magnetic layer thereof has a small thickness because the diameter of the wound tape must be limited to a small value. For this reason, especially, an operation of a time index function (i.e., the function capable of recording an information concerning recording date and time in an empty channel on a magnetic tape, or reproducing the information therefrom) which is generally provided in a micro-cassette tape recorder, is rendered unstable. This is because the time index function is provided by recording the information in the form of a low frequency signal in the order of 30 Hz/-30 dB and therefore highly influenced by a low-frequency sensitivity characteristic of the magnetic tape used.